Metamorphism

Santallier, Danielle; Lardeaux, J. M.; Marchand, J.; Marignac, C.

doi:10.1007/978-3-642-84915-2_28

Cited by 13 publications

(6 citation statements)

References 29 publications

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“…These nappes overlie the Para-Autochthonous units, which are mainly composed of quartzites, micaschists and scarce Ordovician orthogneisses. No early high-pressure relics are observed in the Para-Autochthonous units, which variably underwent lower and upper amphibolite-facies metamorphism (Ledru et al 1989(Ledru et al , 1994Quenardel et al 1991;Santallier et al 1994;Faure et al 2005Faure et al , 2009. The thrusting of the UGU over lower grade units is considered to be responsible for an inverted metamorphic stratigraphy ("inverted metamorphism") which is widespread and typical of several regions in the French Massif Central, such as the Aigurande (Rolin and Quenardel 1980), Marvejols (Briand 1978), Rouergue (Burg et al 1989), Limousin (Friedrich et al 1988;Roig and Faure 2000;Bellot and Roig 2007), Haut-Allier (Marchand 1974;Burg 1977;Burg et al 1984;Schulz et al 1996) and La Sioule (Faure et al 1993;Schulz et al 2001;Schulz 2009).…”

Section: Geological Setting and Sampling Sitesmentioning

confidence: 99%

“…Structural, petrological and geophysical data have shown that the French Massif Central was part of the Variscan tectono-metamorphic history which involved subduction and continental collision of Laurussia with Peri-Gondwana and Gondwana-derived microcontinents (Armorica, Avalonia) in between (Burg et al 1984;Ledru et al 1989;Santallier et al 1994;Matte 2001;Faure et al 2005Faure et al , 2008Faure et al , 2009Kroner and Romer 2013). Five principal nappe units (from top to bottom: Brévenne and Génis, Thiviers-Payzac, Upper Gneiss, Lower Gneiss, and Para-Autochthonous units) have been identified in the internal part of the French Massif Central, as outlined in Ledru et al (1994) and Faure et al (2005Faure et al ( , 2009.…”

Section: Introductionmentioning

confidence: 99%

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Early Carboniferous P-T path from the Upper Gneiss Unit of Haut-Allier (French Massif Central) - reconstructed by geothermobarometry and EMP-Th-U-Pb monazite dating

Schulz¹

2014

Jour. Geosci.

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In the Haut-Allier region (French Massif Central), a Variscan inverted metamorphic sequence is made up by crustal nappes. A high-grade Upper Gneiss Unit (UGU) was thrusted over a Lower Gneiss Unit (LGU) and an amphibolite--facies para-autochthonous Micaschist Unit (MU). Growth-zoned garnets with distinct Mn-Mg-Fe-Ca and trace-element zonation trends occur in kyanite garnet gneisses at Agnat (UGU). The porphyroblasts have been characterised by automated energy-dispersive X-ray spectral mapping with SEM, by electron microprobe and LA-ICPMS analyses. Microstructurally-controlled geothermobarometry based on cation exchange and net transfer reactions was used to reconstruct a syn-deformational and clockwise P-T path for garnet crystallization in the UGU. The P-T path passed maximal pressures at 700 °C/13 kbar and then maximal temperatures at ~800 °C/11 kbar in the stability field of kyanite + K-feldspar. The final stage of the P-T path is a marked decompression from 10 to 5 kbar at 700-750 °C. The timing of this P-T evolution in the UGU has been constrained by electron-microprobe Th-U-Pb monazite dating (CHIME). A detailed interpretation of ages and Y-contents of monazites enclosed in a syncrystalline-rotated Mg-rich garnet allowed to relate a marked pressure decrease along a late stage of the P-T path to ~330 Ma. Simple 1D forward numerical modeling with variations of vertical velocity and geothermal gradient confirmed that the onset of monazite crystallization at ~360 Ma should give a maximal age of the early P-T evolution. A distinct group of Y-rich monazites could be the relic of this Late Devonian event. Most monazite ages from the UGU, the LGU and the MU in the Haut-Allier region range between 360 and 320 Ma, with isochrons for single samples giving ages from 332 to 338 Ma. The prograde-retrograde P-T-t evolution in the UGU appears as an independent Early Carboniferous metamorphic cycle, which was related to a continental collision. It post-dated a Silurian HP-(UHP) event and a subsequent Early Devonian migmatization in the UGU.

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Section: Geological Setting and Sampling Sitesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Early Carboniferous P-T path from the Upper Gneiss Unit of Haut-Allier (French Massif Central) - reconstructed by geothermobarometry and EMP-Th-U-Pb monazite dating

Schulz¹

2014

Jour. Geosci.

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“…The West European Variscan Belt is a collisional belt that experienced crustal thickening (350-310 Ma; Matte, 1986Matte, , 1991 followed by extension and thinning (335-290 Ma;Van den Driessche and Brun, 1989;Faure et al, 1990;Burg et al, 1990;Faure, 1995;Faure et al, 2002). In the French Massif Central, Variscan events range from high-pressure (HP) metamorphism in the Late Silurian-Early Devonian (440-400 Ma) to deposition of late orogenic sediment in the Late Carboniferous-Early Permian (300 Ma) (Pin and Vielzeuf, 1983;Ledru et al, 1989;Santallier et al, 1994;Faure et al, 1997;Matte et al, 1998).…”

Section: Major Tectono-metamorphic Eventsmentioning

confidence: 99%

“…The UGU overlies a lower gneiss unit (LGU) that lacks the HP metamorphic assemblages and is characterized by abundant migmatites. The presence of syn-kinematic cordierite in the migmatitic metasedimentary gneiss of the LGU may be related to high-temperature (HT) decompression of the HP metamorphic assemblages (Mercier et al, 1992;Santallier et al, 1994) under medium-P and high-T conditions (5-6 kbar, 760-780°C (Le Breton and Thompson, 1988)). This high-T metamorphism is dated in the Limousin area at 354.0 ± 4.8 Ma (U-Th/Pb monazite; Gébelin, 2004).…”

Section: Major Tectono-metamorphic Eventsmentioning

confidence: 99%

Syntectonic crustal melting and high-grade metamorphism in a transpressional regime, Variscan Massif Central, France

2009

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Hot collisional orogens are characterized by abundant syn-kinematic granitic magmatism that profoundly affects their tectono-thermal evolutions. Voluminous granitic magmas, emplaced between 360 and 270 Ma, played a visibly important role in the evolution of the Variscan Orogen. In the Limousin region (western Massif Central, France), syntectonic granite plutons are spatially associated with major strike-slip shear zones that merge to the northwest with the South Armorican Shear Zone. This region allowed us to assess the role of magmatism in a hot transpressional orogen. Microstructural data and U/Pb zircon and monazite ages from a mylonitic leucogranite indicate synkinematic emplacement in a dextral transpressional shear zone at 313 +/- 4 Ma. Leucogranites are coeval with cordierite-bearing migmatitic gneisses and vertical lenses of leucosome in strike-slip shear zones. We interpret U/Pb monazite ages of 315 +/- 4 Ma for the gneisses and 316 +/- 2 Ma for the leucosomes as the minimum age of high-grade metamorphism and migmatization respectively. These data suggest a spatial and temporal relationship between transpression, crustal melting, rapid exhumation and magma ascent, and cooling of high-grade metamorphic rocks.;Some granites emplaced in the strike-slip shear zone are bounded at their roof by low dip normal faults that strike N-S, perpendicular to the E-W trend of the belt. The abundant crustal magmatism provided a low-viscosity zone that enhanced Variscan orogenic collapse during continued transpression, inducing the development of normal faults in the transpression zone and thrust faults at the front of the collapsed orogen

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“…In western Europe, the Variscan belt resulted from the collision between Gondwana and Laurussia in the Late Devonian to Early Carboniferous times [ Matte , 1986]. Oceanic and subsequent continental subduction occurred during the Late Silurian‐Devonian [e.g., Santallier et al , 1994; Lardeaux et al , 2001].…”

Section: Geological Settingmentioning

confidence: 99%

A multidisciplinary study of a syntectonic pluton close to a major lithospheric‐scale fault—Relationships between the Montmarault granitic massif and the Sillon Houiller Fault in the Variscan French Massif Central: 2. Gravity, aeromagnetic investigations, and 3‐D geologic modeling

et al. 2008

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[1] New gravity and aeromagnetic investigations have been carried out to understand the emplacement mechanisms of a granitic pluton and the relationships with a nearby lithospheric-scale fault. This paper concerns the second part of a methodological multidisciplinary study and complements previous geochronologic and Anisotropy of Magnetic Susceptibility (AMS) studies on the same pluton. In the northern part of the Variscan French Massif Central (FMC) the Montmarault massif crops out along the Sillon Houiller Fault (SHF). Bouguer and aeromagnetic anomaly maps imply thickening of the pluton along the SHF and suggest laccolitic spreading northwestward. On the basis of petrophysical measurements, direct two-dimensional (2-D) joint gravity and magnetic modeling has been performed along 10 cross sections. In order to quantitatively constrain the 3-D pluton geometry and its relationships with surrounding geologic units, these geophysical cross sections, new structural information (field and AMS measurements), and petrophysical data have been integrated into a regional 3-D geological and geophysical model. Altogether, the results obtained from geochronology, petromagnetic fabrics (part 1), gravity, and aeromagnetic investigations as well as 3-D modeling (part 2) demonstrate that the Montmarault pluton was emplaced during the Namurian period along the SHF. Our results further show that at that time, in response to a NW-SE regional extension, if the SHF existed, it behaved as a normal fault. Mylonites attesting for synmagmatic normal motion on the northeastern part of the Montmarault pluton strengthen this tectonic scenario. During the Late Carboniferous the FMC experienced NE-SW extension along the SHF by 80 km of brittle left lateral wrench offset. This second tectonic event is well recorded in the Stephanian coal basins which were formed along NW-SE listric brittle faults and constrain the present-day shape of the Montmarault pluton.

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Metamorphism

Cited by 13 publications

References 29 publications

Early Carboniferous P-T path from the Upper Gneiss Unit of Haut-Allier (French Massif Central) - reconstructed by geothermobarometry and EMP-Th-U-Pb monazite dating

Early Carboniferous P-T path from the Upper Gneiss Unit of Haut-Allier (French Massif Central) - reconstructed by geothermobarometry and EMP-Th-U-Pb monazite dating

Syntectonic crustal melting and high-grade metamorphism in a transpressional regime, Variscan Massif Central, France

A multidisciplinary study of a syntectonic pluton close to a major lithospheric‐scale fault—Relationships between the Montmarault granitic massif and the Sillon Houiller Fault in the Variscan French Massif Central: 2. Gravity, aeromagnetic investigations, and 3‐D geologic modeling

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